Intransit liquefied gas refrigeration system

ABSTRACT

A liquefied gas vaporizer is provided in the bottom portion of the freeze-sensitive product storage chamber with thermal insulation around the liquid vaporizing conduit and thermally conductive metal floor means contiguously associated with and in heat transfer relation to the thermal insulation.

United States Patent H 1 1 3,714,793

Eigenbrod 1 1 Feb. 6, 1973 [5 INTRANSIT LIQUEFIED GAS I 7 References Cited REFRIGERATION SYSTEM 7 UNITED STATES RATFNTS if [75] filvelliblI-Lgsiel; Eigeifiraa; 'Gfahd 2,479,840 8/1949 Johnson etal ..62/239 x Island NY 2,923,384 2/1960 Black ..62/239 x [73] AssigneeZ ,UniOn Carbide Corporation, New 3,121,999 2/1964 Kasbohm et al ..62/48 York 3,241,329 3/1966 Fntch, Jr. et a1. ...62/239 X 3,421,336 1/1969 Lichtenberger et a1. ..62/239 X [22] Filed: Jan. 18, 1971 [21] A No: 107,541 Primary ExaminerAlbert W. Davis, Jr.

Attorney-Paul A. Rose, Thomas I. OBnen, John C. Related US. Application Data LeFever and Laurence G. Kastriner 63 f S N 777,643, N 21, 1968, 1 555551125? [57] ABSTRACT A liquefied gas vaporizer is provided in the bottom [52] US. Cl. ..62/62, 62/52,6622//56144, portion of the freeze sensitive product Storage chamber with thermal insulation around the liquid 2 "2 g Z g2 aporizing conduit and thermally conductive metal 1 le 0 earc floor means contiguouslyassociated with and in heat transfer relation to the thermal insulation.

16 Claims, 5 Drawing Figures /4 25 Q 2/ Z6 2 5 A Z0 \21\ RY! \L\ \1 j I 14x1 l 1 1 a 8 W 20 24 V /2 E3 /5 J 4 ;L 7/\ (k f\ /6 /6@ /6b /7 /9 PATENIEDFEB 6 ms SHEET 10F 3 FIG./

LN VENTOR. LESTER KURT EIGENBROD W C km ATTORNEY PATENTEDFEB 6 I975 SHEET 2 OF 3 l I 'n.

44 /6c b IXVENTOR.

LESTER KURT EIGENBROD M a fiazah.

ATTORNEY PATENTEU FEB 8 I973 SHEET 3 OF 3 REAR M/DDL E FRONT LOCATION 11v STORAGE CHAMBER LN'VEXTOR. LESTER KURT EIGENBROD ATTORNEY 'uniform storage temperatures INTRANSIT LIQUEFIED GAS REFRIGERATION SYSTEM This application is a continuation of Ser. No. 777,643, filed Nov. 21, 1968, now abandoned.

BACKGROUND OF THE INVENTION temperature range of about 33 to 50F., and when such product is to be refrigerated for a sustained period exceeding about 48 hours. Fresh produce generates internal heat due to respiration, and this heat must be removed by the refrigeration system along with the atmospheric heat unavoidably transferred into the product storage chamber. For such products, satisfactory intransit spray refrigeration for periods exceeding about 2 days can only be provided by maintaining often closely approaching the product freezing point. However, actual freezing must be avoided as it usually causes complete deterioration and an unsalable product.

One of these problems is variation of product temperature within the storage chamber. By way of illustration, consider the liquid nitrogen spray refrigeration of lettuce with a 37-foot overhead spray header and a 35F. thermostat set point. The system described in the Kane et al patent often results in undesirably large temperature gradients between the top and the bottom of the chamber, e.g. 37F. (top) and 50F. (bottom). This problem may not be overcome by simply introducing more refrigeration through the overhead spray conduit,

SUMMARY In the apparatus of this invention, a container holds low-boiling refrigerant liquid as for example liquid nitrogen, and a fluid discharge conduit joins the container to an overhead conduit or header preferably having longitudinally spaced openings for spraying refrigerant fluid into a product storage chamber to maintain the chamber temperature between about 33 and 50F. The improvement comprises liquid discharge conduit means joined to the container and liquid vaporizing conduit means joined to the latter and positioned in the bottom portion of the product storage chamber and extending substantially the length thereof. Thermal insulation is positioned around the liquid vaporizing conduit, and of sufficient effectiveness to control heat conductance (to the refrigerant) to 5-30 Btu' per hour per foot conduit length. Thermally conductive metal floor means are -contiguously associated with and in heat transfer relation to the thermal insulation. The metal floor means extend at least part of the length and width of the product storage chamber.

In a preferred embodiment, gas passageway means are also positioned in the bottom portion-of the product storage chamber extending the length thereof in heat exchange relation with the thermal insulation and the conductive metal floor means, and having opposite ends in open communication with theproduct chamber gas space.

because freeze damage in the top portion of the product load would result without adequately cooling the lowest portion. The natural fluid convection'currents in this region above the product are very concentrated, and the cold refrigerant fluid spray from the openings discharges directly onto the top layer. of product boxes. Moreover,- the problems may not be solved by direct introduction of cold refrigerant in the bottom of the chamber because of resulting product freeze damage in that region.

An object of this invention is to provide an improved system for spray refrigeration of freeze-sensitive product using cryogenic liquid.

Another object is to provide such a system which affords more uniform product temperature from top to bottom of the product storage chamber.

Still another object is to provide improved apparatus for intransit spray refrigeration of freeze-sensitive product which affords more uniform temperature from top to bottom of the product storage chamber at a controllable level between about 33 and 50F.

Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.

In another embodiment a flat plate comprises the thermally conductive metal floor means. In still another embodiment multiple metal channelsare positioned in the bottom of the product storage chamber from endto-end thereof and in contiguous relation to the metal floor means. In this embodiment a first group of channels comprise the gas passageway means, and the liquid vaporizing conduit means are positioned within a second group of channels. The aforementioned thermal insulation occupies the space between the liquid vaporizing conduit means outer wall and the walls of the second group of channels. A flat metal plate joins the lower end of the channels and extends the entire length and width of the product storage chamber as the I,

thermally conductive metal floor. means.

In one method of this invention heat is transferred by solid conduction from the lowest product layer to the aforementioned metal floor means and then from the latter to thermal insulation extending the chamber length beneath the lowest product layer and surrounding the refrigerant liquid vaporizing conduit means. From the thermal insulation the heat is transferred by solid conduction to the conduit means and thence by convection to the refrigerant liquid vaporizing in the transfer relation with the metal floor means. This step is followed by the aforedescribed consecutive solid conduction heat transfers to the thermal insulation and'the refrigerant liquid vaporizing conduit. Finally the heat is transferred by convection from the conduit to the refrigerant liquid vaporizing therein.

As will be discussed hereinafter in detail, this invention achieves a'significant reduction in product temperature difference between top and bottom of the product storage chamber. At the same time the ap- 'paratus facilitates control of the refrigeration temperature at a predetermined level above freezing, i.e.

between about 33 and 50F.

BRIEF DESCRIPTION OF THE DRAWINGS vaporizer-thermal insulation-thermallyconductive flat plate assembly suitable for use in the FIG. 2 embodiment apparatus.

FIG. 4'is a cross-sectional end view of a raised plate and lower flange section floor vaporizer-thermal insulation assembly suitable for use in the FIG. 2 apparatus, and

FIG. 5 is a graph comparing the performance of this invention with a prior art spray refrigeration system.

DESCRIPTION 0F PREFERRED EMBODIMENTS Referring now to the drawings, FIG. 1 illustrates an embodiment in which a truck semi-trailer body constitutes thermally insulated storage chamber 11 for holding freeze sensitive product such as stacked boxes of lettuce 12. This chamber-may be of standard construction for typical mobile refrigerated chambers, e.g. reinforced aluminum siding outer walls, plywood panelled inner walls and plastic foam insulating material between the two walls. That is, the inner wall of the storage chamber is usually constructed of low ther- I mally conductive material. The chamber is closed to the atmosphere but need not be air-tight, as access means such as rear doors (not illustrated) are needed for insertion and removal of product 12. a

A double-walled thermally insulated container 13 is associated with storage chamber 11 for storing pressurized low-boiling liquefied gas having a boiling point at atmospheric pressure below about 20F. The construction of such containers is well-known and is, for example, depicted in' Loveday et al U.S. Pat. No. 2,951,348. Container 13 (either cylindrical or rectangular) is depicted within storage chamber 11 but also may be positioned outside this chamber. Container 13 includes an outer shell completely surrounding an inner storage vessel to form an evacuable insulation space therebetween. This space is preferably filled with an efficient solid thermal insulating material, as for example alternate layers of radiation-impervious barriers such as aluminum foil separated by low conductive fibrous sheeting,,as for example glass fibers. This particular highly efficient insulation is described in U.S. Pat. No.

' high level of insulating quality.

Low-boiling liquefied gases which are suitable for use as refrigerants in the present invention are those which have a boiling point at atmospheric pressure below about -20F. Examples of such liquefied gases are liquid air, liquid argon, liquid carbon dioxide, liquid helium, and liquid nitrogen. Liquid nitrogen is particularly suitable because of its inertness and relative ease of separation from air, and is preferred. While the subsequent discussion refers specifically to nitrogen, it is to be understood that all of the aforementioned gases are suitable-along with mixtures thereof. Although the primary function of storage chamber 11 is to refrigerate the product 12, the preferred liquiiied gases such as nitrogen also control the atmosphere within the chamber and provide an inert blanket surrounding the product.

The vessel within storage container 13 is filled with liquid nitrogen by means well knownto the prior art, such as for example connecting a source of liquid nitrogen stored at above atmospheric pressure to the container. If the liquid nitrogen is stored at a pressure below the operating pressure of container l3,'a suitable pump would be employed and usually additional heat would be added to the pressurized liquid before transferring it into container 13. The liquid nitrogen is preferably charged into container .13 and stored therein at saturated conditions and at temperatures cor-- responding to a vapor pressure above 10 psig. with the entire liquid and vapor substantially in equilibrium. If the aforementioned highly efficient insulation is used, there is no appreciable amount of heat inleak to the inner storage vessel of container 13 and the stored liquid nitrogen is dispensed only by this as-charged vapor pressure. Alternatively the liquid nitrogen may be charged to container 13 under non-saturated conditions and even in the subcooled state. Under these circumstances it would probably be necessary to provide means for building sufficient internal pressure on demand to discharge the liquid. Those skilled in the art will appreciate that this heat maybe introduced externally, using the well-known pressure building coil. The latter includes a liquid discharge conduit, an atmospheric heat vaporizer and a conduit for returning the resulting vapor to the container gas space (not illu'strated). As still another variation known to the art, a

less efficient heatjinaulating material may be used so that sufficient atmospheric heat inleak is available tovaporize sufficient stored liquid refrigerantto form gas pressure to insure liquid discharge on demand.

It is preferred to store the liquid nitrogen refrigerant at pressure below about psig., because higher pressures require impractically small spray conduit openings. Moreover, the inherent lag characteristics of commonly used temperature sensing elements would make adequate control of the liquid refrigerant joined to liquid vaporizing heat exchanger 16 positioned in the bottom portion of product storage chamber 11 and extending substantially the length thereof. Heat exchanger 16 preferably comprises .at least two passes, the first section 16a extending to the opposite end of chamber 11 near the access doors and the second section 16b returning to "the chamber end near liquid storage container 13.

Conduit sections 16a and 16b are surrounded by thermal'insulation 17 having sufficient effectiveness to control heat conductance to 5-30 Btu per hour per foot conduit length. Thermally conductive metal floor means 18 is contiguously associated with and in heat transfer relation to thermal insulation 17. Metal floor means 18 extend at least part of the length and at least part of the width of the product storage chamber beneath the conduit 16 thermal insulation 17 assembly, and is illustrated as a flat plate. Product platform 19 is superimposed on the heat exchanger 16 thermal insulation 17 metal floor member 18 assembly, and product 12 is positioned thereon.

The vaporized and normally superheated gas discharged from heat exchanger 16 is released into the product storage chamber 11 through outlet 20, preferably located in the upper portion ofchamber 11. in this manner the remaining sensible refrigeration of this gas is recovered by contact with the stored product 12. To avoid overpressurization of the chamber, part of this gas may be discharged through vent 20a to the environment outside chamber 11. As an alternative to direct introduction through outlet 20', the refrigerant gas discharged from conduit 16 may be introduced to the overhead spray header upstream of the openings for discharge therethrough into chamber v11.

Fluid-discharge conduit 21 communicates at one end with liquid refrigerant storage container 13. As illustrated conduit 21 branches from liquid discharge conduit 14, but alternatively the former could separately extend into container'l3. Overhead spray conduit 22 joins the other end of conduit 21 and extends substantially the entire length of product chamber 11, with openings 23 spaced along the length for discharging refrigerant fluid therein. Openings 23 may be oriented either horizontally or slightly downwardly in the circumference of conduit 22, and preferably have a circular configuration for uniform symmetrical discharge of sprayed refrigerant. Thermal insulation may be provided around the outer surface of spray conduit 22 if desired.

Cold fluid flow control means are provided including a temperature sensing element 24 such as a bulb positioned within the storage chamber 11 gas space. This bulb 24 is connected by signal receiving means 25 to temperature controller 26, and signal transmitting means 27 provides-communication between the controller and control valve 28 in fluid discharge conduit 21. The flow control means may be electrically or pneumatically operated.

ln normaloperation, refrigerant liquid is continuously dispensed from storage container 13 through discharge conduit 14 and control valve 15 tofloor vaporizer 16. Thisliquid is at least vaporized therein by surrounding heat, i.e. from the product chamber environment gas, from the product itself through the solid conductive heattransfer path joining the product and from heat inleak through the floor from the environment surrounding chamber 11. Under normal conditions the vaporized liquid is at-least partially superheated in floor vaporizer 16 so' that the gas released through outlet 20 is at an intermediate temperature between the liquid and the product chamber, e.g. about l0F. to 15F. for nitrogen. The remaining sensible refrigeration of this cold gas is then transferred to the product as the gas circulates in the chamber around and in contact with the product. The dispensing vapor pressure in liquid container 13 should be sufflcient to obtain the necessary gas circulation in chamber 11. Cold fluid, normally liquid, is intermittently dispensed from liquid refrigerant storage container 13 through conduits 21 and 22' in response to signals from temperature bulb 24, as part of the refrigeration required to maintain the sensed temperature within the predetermined range, e.g. 33 to 50F.

FIG. 2 illustrates a preferred embodiment in which the floor vaporizer outlet 20 joins gas expander 30 positioned in the upper portion of chamber 11 beneath spray conduit 22 and preferably near one end thereof. Expander 30 may for example be a commercially available sliding vane-type air motor with an inlet pressure of about 10-25 psig. operating at 200-1 ,500 rpm. or greater, but a turbine type expander may be used instead. Expander 30 is joined by shaft coupling means 31 to fan 32 so that the energy released by expansion of the pressurized gas from higher to lower superatmospheric pressure is recovered as rotational energy to drive the fan. The latter serves to improve circulation of gas within chamber 11 and reduce end-to-end temperature differences. If desired, the vaporized refrigerant gas may be further warmed before passing through expander 30 by flow through a superheater outside chamber 11. The broad idea of a gas-driven fan in an intransit spray refrigeration system is not part of the present invention, but is described and claimed in United States application Ser. No. 643,709 filed June 5, 1967 in the names of H. W. Lichtenberger and D. P.

Maurer and issued Jan. 14, 1969 as U. S. Pat. No.

In the floor vaporizer 16 of the FIG. 2 embodiment, separate passageways 16c are provided for end-to-end flow of the circulating'chamber environment gas in heat exchange relation with the vaporizing refrigerant liquid and product 12. The warmed chamber environment gas emerging from the opposite end of passageway 16c flows upwardly behind bulkhead 33 and is at least partially recirculated by fan 32. The later also insures circulation of the expanded gas discharged from expander 30 through port 34. If desired, a portion of the recirculating gas may be discharged to the atmosphere through vent 20a.

Fan 32 may alternatively be electrically driven from a power source, such as a battery and generator unit 32a joined to the fan by wires 32b. For such electrically-driven fans, the gas from the floor vaporizer outlet is usually dispensed in the vicinity of the fan for effective circulation. The advantage of the gas-driven fan for the FIG. 2 embodiment is that external power is not required. The energy to operate fan 32 is derived from the on-board pressurized refrigerant fluid itself.-

Another feature of the FIG. 2 embodiment is a second temperature responsive control system for refrigerant liquid .in discharge conduit 14 from container 13. Temperature sensing element 24a, as for example a thermostat bulb, is positioned in the bottom portion of chamber 11 in thermal association with a passageway 160 for the circulating environment gas. This element may be located in the passageway gas space or against the passageway wall. The element 24a is connected by signal receiving means 25a to temperature controller 260, and signal transmitting means 27a provides communication between the controller and control valve 15 in liquid discharge conduit 14. The flow control means may be electrically or pneumatically operated. The temperature response range of element 24a depends on the product but in any event is between about 33 and 50F. With lettuce the setting is preferably 2-3F. colder than the temperature setting for overhead bulb 24. The setting would beconside-rably higher for products having optimum refrigeration levels of 4O4SF.

FIG. 3 illustrates a preferred floor vaporizer 16 construction suitable for use in the FIG. 2 railcar embodiment, including multiple metal channels 35 positioned in the bottom of the storage chamber 11 from end-toend thereof. The FIG. 3 construction is also suitable for use in highway truck and semi-trailer embodiments of the invention. A first group of channels 35 are open at opposite ends and form the passageways 16c for circulating chamber environment gas. It should be appreciated, however, that the channel-type construction may also be employed for heat exchangers 16 not including gas passageways 160, in accordance with the FIG. 1 embodiment. The refrigerant liquid vaporizing conduits 16a are positioned within a second group of channels 35 and thermal insulation 17 substantially fills the space between the conduit outer wall and the channel wall. Superheating conduits 16b are similarly positioned within another second group of channels 35 and second thermal insulation 17b substantially fills the space between the conduit outer wall and the channel wall.

it has been previouslyindicated that thermal insulation 17 and 17b is of sufficient effectiveness to control heat conductance 'to 5-30 Btu per hour per foot conduit length to the vaporizing refrigerant liquid. The heat conductance required for a particular system depends on several variables, one being the number of conduits extending from end-to-end of chamber'll and comprising heat exchanger 16. In general, a, larger number of conduits permits lower heat conductance due to the availability of more heat transfer paths. Conversely, if only two conduits are employed as for example liquid vaporizing conduit 16a and superheating conduit l6b, all of the floor refrigeration needed by the product must be provided thereby and the heat conductance should be relatively high.

Another variable affecting the heat conductance required for thermal insulation 17 and 17b is the heat conduction of metal floor means 18. if more heat transfer paths are provided by means 18 their spacing from each other is reduced and their heat conduction may be lower, e.g. thinner members or members constructed of lower thermal conductivity material. Still another variable affecting the required heat conductance of the thermal insulation 17 and 17b is the quality of thermal insulation used in the chamber 11 walls and floor. Ineffective chamber insulation requires the floor heat exchanger 16 to provide more total refrigeration, which may require more refrigerant fluid passages or metal floor means of higher conduction.

A further consideration which affects the heat conductance required for thermal insulation 17 and 17b is the use of environment gas passageways 160. If environment gas is circulated through heat exchanger 16 as for example illustrated in the FIG. 3 embodiment, a relatively higher heat conductance is satisfactory. This is because the circulating gas receives a portion of the refrigeration by gas conduction and transfers same to the product. Accordingly, the refrigeration transferred to the product by solid conduction is decreased and a higher overall heat conductance for the thermal insulation may be tolerated.

Heat conductances below about 5 Btu per hour per foot conduit length would prevent adequate heat transfer to vaporize the refrigerant liquid in the conduit, and heat 'conductances above about 30 would permit sufficient heat transfer to cause possible freeze damage of the lowest layer of stored product. That is, using a cryogenic refrigerant such as liquid nitrogen in conduit 16a surrounded by thermal insulation having heat conductance above about 30 Btu/hr. ft. would cool the outer portion of the lowest product to near or below 32F.

1 When gas passageways are provided in heat exchanger 16 for the circulation of environment gas from end-to-end, the heat conductance of thermal insulation 17 and 17b is preferably 10 to 258m per hour per foot conduit length for the aforementioned reasons. When such gas passageways are not provided, as for example in the FIG. 1 embodiment, the heat conductance is preferable 5 to 10 Btu per hour per foot conduit length.

The thickness of a particular thermal insulation required to provide a desired heat conductance depends on the materials thermal conductivity A thin layer of relatively efficient thermal insulation could provide the same heat conductance as a thicker'layer of less efficient material. 1

' In one successful embodiment of this invention, vaporizing conduit 16a was formed of 5/l6-inch O.D. X0.030-inch thick copper tubing, positioned within 2- inch deep aluminum channels and surrounded by foamed-in-place closed cell polyurethane plastic insulation having heat conductance of about 15 Btu/hr. ft. conduit length. There were no frost spots on product support base. in another embodiment, 1% inch diameter cylindrical sections of nitrile rubber foam insulation with longitudinal slits were positioned around the same size copper tubing. This insulation afforded a heat conductance of about 22 Btu/hr. ft. conduit length and was satisfactory, although frost spots occasionally appeared A series of tests were conducted which illustrate the qualitative advantages of this invention as compared to a similar nitrogen spray refrigeration system without the floor vaporizer or an overhead fan. In this prior art system hereinafter designated as system A, lettuce was shipped in a highway trailer from Salinas, California to Chicago, Illinois. The trailer was 40 feet long, 8 feet wide and was refrigerated by saturated liquid nitrogen discharged from the storage container at about 20 psig. vapor pressure through the overhead spray conduit. The temperature control set point was 35F. for system A and the succeeding systems.

In system B the same type of trailer was used as in system A, but a channel-type floor vaporizer as illustrated in FIG. 3 was employed with a circulating fan positioned in the upper portion of the chamber and beneath the spray header as illustrated in FIG. 2. Both centrifugal and axial flow type fans were tested; in each instance the fan was coupled to an airmotor driven by expanding nitrogen vapor (also illustrated in FIG. 2). More specifically, the floor vaporizer comprised /4 and /l6-inch O.D. copper and aluminum tubing supported by /;-inch diameter stainless steel wire clips at spaced intervals within 2-inch deep aluminum floor channels spaced about 2 inches apart and joined on the bottom by an integral floor member of about 3/l6-inch thickness. These support clips were initially longitudinally spaced about 2 feet apart in the coldest portion of the vaporizing section, with increasingly'closer spacing down to about 3 inches apart at the outlet end of heat exchanger 16. The space surrounding the refrigerant tubing within the channel was filled with foamed-in-place closed cell polyurethane plastic insulation. Two complete passes of insulated refrigerant tubing were used, each extending the length of the trailer floor with heat conductance of-about l5 Btu/hr. ft. tubing length in both the nitrogen liquid and vaporcarrying passageways. About 8 lb./hour of nitrogen was passed through the floor vaporizer in the 40-foot long by 8-foot wide trailer. This flow was sufficient to cool the product at the lowest level within the chamber and also develop power needed to operate the aforemen-' tioned fans.

The air motor used in system B was the sliding vane type suitable for 10-100 psig. inlet pressure, and rated at 1,600 rpm. at psig. inlet pressure and atmospheric exhaust pressure. The centrifugal fan was rated for delivering 660 cfm. air flow against a static pressure of 0.3 inches water pressure at 1,000 rpm. The motor was operated at about 225 rpm. and coupled through spurgears to the fan operated at about 500 rpm. to deliver 300 cu. ft. per min. gas against 0.05 inches water head pressure. The-axial flow propeller-type fan (substituted for the centrifugal fan) was 10 inches in diameter with four blades and capable ofdelivering 600 cfm. air flow against a static pressure of 0.05 inches water pressure at 1,500 rpm. The motor and fan were directly coupled and both run at about 1,500 rpm. In the tests involving system B the trailer was stationary, but boxed lettuc was used as the stored product.

The performance of the two systems is summarized in the FIG. 5 graph showing the temperature difference (AT) between the thermostat set point temperature in the gas space above the top layer of product, and the temperature of product about 6 inches above the product support platform for various longitudinal locations within the storage chamber. The temperatures were obtained from thermocouples placed in the product at the front, middle and rear of the chamber and continuously recorded during each test. Although the data is not susceptible to quantitative comparison because the test conditions and equipment were not identical, systems A and B may be qualitatively compared. The system B data is represented as a band because the AT was also affected by the circulation rate of environment gas through the floor vaporizer. In general the AT was reduced as this circulation rate increased from about 300 cfh. to 600 cfh. It is apparent that substantial improvement in reduced variation of AT from the chamber top-to-bottom and end-to-end was achieved with system B. For example, in system B the end-to-end product temperatures were all maintained within 25F. above the bulb set point temperature. It is also apparent that the AT was much smaller for system B than system A. For example, while system B maintained the lowest product layer within 25F. above the set point temperature as required by the method, of this invention, system A permitted the temperature of most of the lowest product layer to rise more than 8F. above the set point and a portion of this product to rise up to 15F. above the set point temperature.

Although preferred embodiments ofthe invention have been described in detail, it is contemplated that modifications of the method and apparatus may be made and that some features may be employed without others, all within the spirit and scope of the invention; For example, this refrigerating system may also be used with stationary equipment.

What is claimed is:

1. In apparatus for spray refrigeration of freeze-sensitive product including a container for holding low-boiling refrigerant liquid, a fluid discharge conduit joined to said container and an overhead conduit having openings for refrigerant spraying into a product storage chamber to maintain the chamber temperature between about 33 and 50F., the improvement comprising liquid discharge conduit means joined to said container, liquid vaporizing conduit means joined to said liquid discharge conduit means positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof, thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to control heat conductance to 5-30 Btu per hour per foot conduit length, and thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation, said metal floor means extending at least part of the length and at least part of the width of I said product storage chamber inside the chamber and beneath the product.

2. In apparatus for spray refrigeration of freeze-sensitive product in a storage chamber including a container for holding low-boiling refrigerant liquid, at fluid discharge conduit joined to said container and an over,- head spray conduit having longitudinally spaced openings for refrigerant spraying into said product storage chamber to maintain the chamber temperature between about 33 and 50F., the improvement comprising liquid discharge conduit means joined to said In normal operation the vaporizing liquid is also superheated in heat exchanger 16, and the temperature difference between refrigerant and product diminishes in the refrigerant superheating section as compared with the vaporizing section. (For example, the temperature difference between vaporizing liquid nitrogen refrigerant and the product is about 350F., while thetemperature difference between the superheated nitrogen gas refrigerant and the product may be as small as F.) Accordingly, if the same thermal insulation is used in the superheating section the heat conductance will diminish and the end-to-end heat transfer rate within the product chamber would be uneven. To

avoid this situation, thermally conductive metal means may be longitudinally spaced in at least the superheating section of heat exchanger 16, in thermal association with the refrigerant conduit 16b and the thermally conductive metal floor means 18. As illustrated in FIG. 3, metal clips 36 positioned around conduit 16b with arms 37 bearing against the heat exchanger walls at varying longitudinal intervals may comprise the thermally conductive metal means. Other means, as for example metal washers, could be used as the controlled thermal contacts between the refrigerant conduit and the metal floor means. I

It should be understood that the required heat conductance range of about 5-30 Btu/hr. ft. conduit in the liquid refrigerant vaporizing section includes the conductance due to longitudinally spaced thermally conductive metal means if such are used. In this event, the heat'conductance would be the sumof the contributions from the thermal insulation and the conductive metal means.

In the FIG. 3 embodiment, metal clips 36-37 perform not only the aforedescribed thermal conduction function but also structurally support and position conduit 16b within the channels. Alternatively, separate elements could be employed for each function. ln this event the conduit support elements might be formed of non-conductive material such as high-strength plastic, and the thermal conductors could be metal washers surrounding conduit 16b but of sufficient diameter to touch the walls of the channels 35. Flat metal plate 18 joining the lower ends of channels 35 and extending the entire length and width of productstorage chamber 11 comprises the thermally conductive metal floor means. In this embodiment, plate 18 is integral with the channels and provides a highly effective path for heat transfer across the width of product chamber 11. That is, heat carried into passageways 160 by the environment gas is transferred by plate 18 and clips 36 to the refrigerant in conduits-16a and 16b. This lateral heat transfer is in addition to that occurring through thermal insulation 17, the latter being essential to the invention.

The amount of heat laterally transferred by metal plate 18 is of course dependent on the plate thickness and thermal conductivity. In one successful embodiment employing the integral channel-plate construction of FIG. 3, the lower floor plate element is formed of 3/l6-inch thick aluminum. It will be recognized that relatively thick metal floor members permit the use of thermal insulations having relatively higher heat conductances.

Product support member 38 is preferably positioned over the top of the thermally insulated refrigerant conduit sections of heat exchanger 16 .to prevent debris from accumulating in thermal insulation 17 and mechanical damage to the heat exchanger. Member 38 may be constructed of any convenient material, e.g." light metal sheeting or plywood.

FIG. 4 illustrates another heat exchanger 16 metal floor means 18 embodiment which is particularly suitable for railroad cars having a pallet-type wooden support construction for the product. The latter consists of longitudinal stringers 40 and overlying cross planking 41 positioned to form longitudinal spaces. A single refrigerant liquid vaporizing passageway 16a and two refrigerant vapor superheating passageways 16b are employed, with each'passageway surrounded by thermal insulation 17:: or 17b. In this embodiment the same type of thermal insulation material is employed for both passageways, but different insulation thicknesses are used to' provide about the same thermal conductance throughout the length of, heat exchanger 16. In particular, a relatively thicker insulation layer 17a is provided around vaporizing passageway 16a because of the larger AT, and a relatively thinner insulation layer 17b is positioned around superheating passageways 16b because of the lower AT between refrigerant vapor and the product. If desired, a larger number of refrigerant fluid passageways could be used so that the desired heat conductancefor insulation m and 1712 would be lower. I

The refrigerant liquid-vapor conduit and thermal insulation assemblies of FIG. 4 are each covered by inverted trough members 42 with lower flange sections 44 and 44b positioned against the product chamber bottom and extending transverse thereto. As illustrated lower flange sections 44 and 44b have widths less than the chamber bottom width, but provide a lateral path for heat conduction from the chamber environment gas passageways 16c to the refrigerant fluid in conduits 16a and 16b.

Another method of this invention may be practiced with the FIG. 4 apparatus, wherein the circulating environment gas, after convective heat exchange with the warmer product, is circulated by fan 32 in heat transfer relation with metal floor means 44 and 44b by flow through gas passageways 16c. The heat received in this manner bythe metal floor means 44 and 44b is then transferred by solid conduction to thermal insulation 17a and 17b surrounding refrigerant liquid vaporizing and superheating conduits 16a and l6b respectively (see FIG. 4). The heat is next transferred by solid conduction from the thermal insulation to conduits 16a and 16b and thence by convection to the refrigerant fluid therein. These heat transfers are sufficient to maintain the lowest product layer from end-to-end of chamber 11 within 5F. of the chamber gas temperature as sensed by bulb 24. v

container, liquid vaporizing conduit means joined to said liquid discharge conduit means positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof, thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to control heat conductance to -25 Btu per hour per foot conduit length, gas passageway means also positioned in the bottom portion of said product storage chamber extending substantially the length thereof in heat exchange relation with said thermal insulation and having opposite ends in open communication with the product chamber gas space, and thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation and said gas passageway means, said metal floor means extending at least part of the length and at least part of the width of said product storage chamber inside the chamber and beneath the product.

3. Apparatus for intransit spray refrigeration of freeze-sensitive product comprising: i

a. a storage chamber for said product;

b. a thermally insulated container associated with the storage chamber for cold pressurized low-boiling liquefied gas have a boiling point at atmospheric pressure below about -F.;

c. fluid discharge means joined at one end to said container;

spray conduit means joined at one end to the other end of said fluid discharge means, positioned within theupper portion of said storage chamber and extending substantially the entire length thereof with openings spaced alongthe length for discharging discrete streams of cold fluid into the storage chamber to refrigerate said product;

e. cold fluid flow control means comprising a first temperature sensing element positioned within saidstorage chamber and a control valve operably interposed in said fluid discharge conduit being connected to said first temperature sensing element to be responsive to the storage chamber temperature as sensed by such first element to maintain said chamber at between about 33 and 50F.;

f. liquid discharge conduit means joined at one end to said container;

g. liquid vaporizing conduit means having an inlet end joined to the other end of said liquid discharge conduit means, positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof;

opposite ends in open communication with the product chamber gas space, and wherein said thermal insulation is of quality to provide heat conductance of 10 to 25 Btu per hour per foot conduit length.

5. Apparatus according to claim 4 having multiple metalchannels positioned in the bottom of said product storage chamber from end-to-end thereof with a first group of said channels comprising said gas passageway means, with said liquid vaporizing conduit means positioned within a second group of said channels, and said thermal insulation filling the space between the liquid vaporizing conduit means outer wall and the walls of the second group of channels, and wherein a flat metal plate joining the lower ends of said channels and extending the entire length and width of said product storage chamber comprises said thermally conductive metal floor means.

6. Apparatus according to claim 3 in which a flat plate comprises said thermally conductive metal floor means. i

7. Apparatus according to claim 3 in which a multiplicity of raised plates having lower flange sections positioned against the chamber floor comprise said thermally conductive metal floor means.

8. Apparatus according to claim 4 in which a fan is positioned within the upper portion of said storage chamber at an end thereof and beneath said spray conduit means.

9. Apparatus according to claim 8 including electric power means joined to said fan.

10.' Apparatus according to claim 3 including cold gas superheating conduit means having an inlet end joined to the discharge end of said liquid vaporizing conduit means and positioned in the bottom portion of said product storage chamber and oriented parallel to said liquid vaporizing conduit; second thermal insulation around said superheating conduit of sufficient effectiveness to control heat conductance to 5-30 Btu per hour per foot conduit length; and second thermally superheating conduit means for expanding the vaporized liquid and being powered solely by such expansion to produce rotational energy, and a fan positioned within the upper portion of said storage h. thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to provide heat conductance of 5 to 50 Btu per hour per foot conduit length; and

i. thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation, extending at least part of the lengthand at least part of the width-of said product storage chamber inside the chamber and beneath the product.

4. Apparatus according to claim 3 with gas passageway means positioned in the bottom of said product storage chamber extending substantially the length thereof in heat exchange relation with said thermal insulation and said metal floor means, and having chamber at an end thereof beneath said spray conduit means and joined to said gas expander for receiving said rotational energy.

12. Apparatus'according to claim 3 including a multiplicity of longitudinally spaced thermally conductive metal means associated with said thermal insulation and said metal floor means.

13. Apparatus for intransit spray refrigeration of freeze-sensitive product comprising:

a. a storage chamber for said product;

b. a thermally insulated container associated with the storage chamber for cold pressurized low-boiling liquefied gas having a boiling point at atmospheric pressure below about 20F.;

c. fluid discharge means joined at one end to said container;

. spray conduit means joined at one end to the other end of said fluid discharge means, positioned within the upper portion of said storage chamber and extending substantially the entire length thereof with openings spaced along the length for discharging discrete streams of cold fluid into the storage'chamber to refrigerate said product;

. cold fluid flow control means comprising a first temperature sensing element positioned within said storage chamber and a control valve operably interposed in said fluid discharge conduit being connected to said first temperature sensing elemerit to be responsive to the storage chamber temperature as sensed by such first element to maintain said chamber at between about 33 and 50F.; f. liquid discharge conduit means joined at one end to said container;

liquid vaporizing conduit means having an inlet end joined to the other end of said liquid discharge conduit means, positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof;

h. thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to provide heat conductance of 10 to Btu per hour per foot conduit length;

. thermally conductive metal floor means contiguouslyassociated with and in heat transfer relation to said thermal insulation, extending at least part of the length and at least part of the width of said product storage chamber inside the chamber and beneath the product; I

j. gas passageway means positioned in the bottom of said product storage chamber extending substantially the length thereof in heat exchange relation with said thermal insulation (h) and said metal floor means (i), and having opposite ends in open communication with the product chamber gas space; and

k. liquid discharge flow control means comprising a second temperature sensing element positioned in the bottom portion of said chamber in thermal association with said gas passageway means (j) and a control valve operably interposed in said liquid discharge conduit (f) being connected to said second temperature sensing element to be responsive to the gas passageway temperature as sensed by such element to maintain such temperature at between about 33 and 50F.

14. In a method for intransit spray refrigeration of freeze-sensitive, product in a storage chamber by passing low-boiling refrigerant liquid from a container through longitudinally spaced openings in a spray conduit overhead said product and in response to sensing conduit means transferrin said heat by solidconduction from said thermal insu ation to said conduit means,

and finally transferring said heat by convection from said conduit means to the refrigerant liquid vaporizing therein, the heat transfers being sufficient to maintain the lowest product layer from end-to-end of said chamber within 5F. of the sensed chamber gas temperature. 7

15. A method according to claim 14 in which cold gas within the product chamber is continuously circulated around said product in convective heat transfer relation therewith, and thereafter circulated in end-toend convective heat transfer relation with said metal floor means beneath thelowest product layer.

16. In a method for intransit spray refrigeration of freeze-sensitive product in a storage chamber by passing low-boiling refrigerant liquid from a container through longitudinally spaced openings in a spray conduit overhead said'product and in response to sensing of the chamber gas temperature above said product to maintain said gas temperature-between about 33 and 50F., the improvement of continuously circulating cold gas in convective heat transfer relation with the warmer product, circulating the warmed gas in heat transfer relation with metal floor means inside the chamber extending at least part of the chamber length and at least part of the chamber width beneath the lowest product layer, then transferring said heat by solid conduction from said metal floor means to thermal insulation inside the chamber extending the chamber length beneath said lowest product layer and surrounding refrigerant liquid vaporizing conduit means, transferring said heat by solid conduction from said thermal insulation to said conduit means, and finally transferring said heat by convection from said conduit means to the refrigerant liquid vaporizing therein, the heat transfers being sufficient to maintain the lowest product layer from end-to-end of said vchamber within 5F. of the sensed chamber gas temperature. 

1. In apparatus for spray refrigeration of freeze-sensitive product including a container for holding low-boiling refrigerant liquid, a fluid discharge conduit joined to said container and an overhead conduit having openings for refrigerant spraying into a product storage chamber to maintain the chamber temperature between about 33* and 50*F., the improvement comprising liquid discharge conduit means joined to said container, liquid vaporizing conduit means joined to said liquid discharge conduit means positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof, thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to control heat conductance to 5-30 Btu per hour per foot conduit length, and thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation, said metal floor means extending at least part of the length and at least part of the width of said product storage chamber inside the chamber and beneath the product.
 1. In apparatus for spray refrigeration of freeze-sensitive product including a container for holding low-boiling refrigerant liquid, a fluid discharge conduit joined to said container and an overhead conduit having openings for refrigerant spraying into a product storage chamber to maintain the chamber temperature between about 33* and 50*F., the improvement comprising liquid discharge conduit means joined to said container, liquid vaporizing conduit means joined to said liquid discharge conduit means positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof, thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to control heat conductance to 5-30 Btu per hour per foot conduit length, and thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation, said metal floor means extending at least part of the length and at least part of the width of said product storage chamber inside the chamber and beneath the product.
 2. In apparatus for spray refrigeration of freeze-sensitive product in a storage chamber including a container for holding low-boiling refrigerant liquid, a fluid discharge conduit joined to said container and an overhead spray conduit having longitudinally spaced openings for refrigerant spraying into said product storage chamber to maintain the chamber temperature between about 33* and 50*F., the improvement comprising liquid discharge conduit means joined to said container, liquid vaporizing conduit means joined to said liquid discharge conduit means positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof, thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to control heat conductance to 10-25 Btu per hour per foot conduit length, gas passageway means also positioned in the bottom portion of said product storage chamber extending substantially the length thereof in heat exchange relation with said thermal insulation and having opposite ends in open communication with the product chamber gas space, and thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation and said gas passageway means, said metal floor means extending at least part of the length and at least part of the width of said product storage chamber inside the chamber and beneath the product.
 3. Apparatus for intransit spray refrigeration of freeze-sensitive product comprising: a. a storage chamber for said product; b. a thermally insulated container associated with the storage chamber for cold pressurized low-boiling liquefied gas have a boiling point at atmospheric pressure below about -20*F.; c. fluid discharge means joined at one end to said container; d. spray conduit means joined at one end to the other end of said fluid discharge means, positioned within the upper portion of said storage chamber and extending substantially the entire length thereof with openings spaced along the length for discharging discrete streams of cold fluid into the storage chamber to refrigerate said product; e. cold fluid flow control means comprising a first temperature sensing element positioned within said storage chamber and a control valve operably interposed in said fluid discharge conduit being connected to said first temperature sensing element to be responsive to The storage chamber temperature as sensed by such first element to maintain said chamber at between about 33* and 50*F.; f. liquid discharge conduit means joined at one end to said container; g. liquid vaporizing conduit means having an inlet end joined to the other end of said liquid discharge conduit means, positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof; h. thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to provide heat conductance of 5 to 50 Btu per hour per foot conduit length; and i. thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation, extending at least part of the length and at least part of the width of said product storage chamber inside the chamber and beneath the product.
 4. Apparatus according to claim 3 with gas passageway means positioned in the bottom of said product storage chamber extending substantially the length thereof in heat exchange relation with said thermal insulation and said metal floor means, and having opposite ends in open communication with the product chamber gas space, and wherein said thermal insulation is of quality to provide heat conductance of 10 to 25 Btu per hour per foot conduit length.
 5. Apparatus according to claim 4 having multiple metal channels positioned in the bottom of said product storage chamber from end-to-end thereof with a first group of said channels comprising said gas passageway means, with said liquid vaporizing conduit means positioned within a second group of said channels, and said thermal insulation filling the space between the liquid vaporizing conduit means outer wall and the walls of the second group of channels, and wherein a flat metal plate joining the lower ends of said channels and extending the entire length and width of said product storage chamber comprises said thermally conductive metal floor means.
 6. Apparatus according to claim 3 in which a flat plate comprises said thermally conductive metal floor means.
 7. Apparatus according to claim 3 in which a multiplicity of raised plates having lower flange sections positioned against the chamber floor comprise said thermally conductive metal floor means.
 8. Apparatus according to claim 4 in which a fan is positioned within the upper portion of said storage chamber at an end thereof and beneath said spray conduit means.
 9. Apparatus according to claim 8 including electric power means joined to said fan.
 10. Apparatus according to claim 3 including cold gas superheating conduit means having an inlet end joined to the discharge end of said liquid vaporizing conduit means and positioned in the bottom portion of said product storage chamber and oriented parallel to said liquid vaporizing conduit; second thermal insulation around said superheating conduit of sufficient effectiveness to control heat conductance to 5-30 Btu per hour per foot conduit length; and second thermally conductive metal floor means contiguously associated with and in heat transfer relation to said second thermal insulation, extending at least part of the length and at least part of the width of said product storage chamber beneath the product.
 11. Apparatus according to claim 10 including a gas expander joined to the discharge end of said cold gas superheating conduit means for expanding the vaporized liquid and being powered solely by such expansion to produce rotational energy, and a fan positioned within the upper portion of said storage chamber at an end thereof beneath said spray conduit means and joined to said gas expander for receiving said rotational energy.
 12. Apparatus according to claim 3 including a multiplicity of longitudinally spaced thermally conductive metal means associated with said thermal insulation and said metal floor means.
 13. Apparatus for intransit spray refrigeratioN of freeze-sensitive product comprising: a. a storage chamber for said product; b. a thermally insulated container associated with the storage chamber for cold pressurized low-boiling liquefied gas having a boiling point at atmospheric pressure below about -20*F.; c. fluid discharge means joined at one end to said container; d. spray conduit means joined at one end to the other end of said fluid discharge means, positioned within the upper portion of said storage chamber and extending substantially the entire length thereof with openings spaced along the length for discharging discrete streams of cold fluid into the storage chamber to refrigerate said product; e. cold fluid flow control means comprising a first temperature sensing element positioned within said storage chamber and a control valve operably interposed in said fluid discharge conduit being connected to said first temperature sensing element to be responsive to the storage chamber temperature as sensed by such first element to maintain said chamber at between about 33* and 50*F.; f. liquid discharge conduit means joined at one end to said container; g. liquid vaporizing conduit means having an inlet end joined to the other end of said liquid discharge conduit means, positioned inside the bottom portion of said product storage chamber and extending substantially the length thereof; h. thermal insulation around said liquid vaporizing conduit of sufficient effectiveness to provide heat conductance of 10 to 25 Btu per hour per foot conduit length; i. thermally conductive metal floor means contiguously associated with and in heat transfer relation to said thermal insulation, extending at least part of the length and at least part of the width of said product storage chamber inside the chamber and beneath the product; j. gas passageway means positioned in the bottom of said product storage chamber extending substantially the length thereof in heat exchange relation with said thermal insulation (h) and said metal floor means (i), and having opposite ends in open communication with the product chamber gas space; and k. liquid discharge flow control means comprising a second temperature sensing element positioned in the bottom portion of said chamber in thermal association with said gas passageway means (j) and a control valve operably interposed in said liquid discharge conduit (f) being connected to said second temperature sensing element to be responsive to the gas passageway temperature as sensed by such element to maintain such temperature at between about 33* and 50*F.
 14. In a method for intransit spray refrigeration of freeze-sensitive product in a storage chamber by passing low-boiling refrigerant liquid from a container through longitudinally spaced openings in a spray conduit overhead said product and in response to sensing of the chamber gas temperature above said product to maintain said gas temperature between about 33420 and 50*F., the improvement of transferring heat by solid conduction from the lowest product layer to metal floor means inside the chamber extending at least part of the chamber length and at least part of the chamber width beneath said lowest product layer, then transferring said heat by solid conduction from said metal floor means to thermal insulation inside the chamber extending the chamber length beneath said lowest product layer and surrounding refrigerant liquid vaporizing conduit means, transferring said heat by solid conduction from said thermal insulation to said conduit means, and finally transferring said heat by convection from said conduit means to the refrigerant liquid vaporizing therein, the heat transfers being sufficient to maintain the lowest product layer from end-to-end of said chamber within 5*F. of the sensed chamber gas temperature.
 15. A method according to claim 14 in which cold gas within the product cHamber is continuously circulated around said product in convective heat transfer relation therewith, and thereafter circulated in end-to-end convective heat transfer relation with said metal floor means beneath the lowest product layer. 